The present disclosure relates to semiconductor structures, and particularly to a metal-oxide-semiconductor field effect transistor (MOSFET) having a metal gate and methods of manufacturing the same.
Controlling the threshold voltage of a metal-oxide-semiconductor field effect transistor (MOSFET) is one of the challenges in manufacturing a metal gate MOSFET. Especially, providing a low threshold voltage for a p-type MOSFET having a metal gate has proven to be difficult for the gate first integration scheme, i.e., the conventional integration scheme in which the gate material is not subsequently replaced.
The gate last integration scheme that employs a replacement gate remains an alternative. However, obtaining a material that provides effective work function corresponding to the valence band edge of silicon is still challenging.
Efforts to alter the work function of a metal layer by conventional thermal oxidation have resulted in an increase in effective oxide thickness (EOT), which degrades the performance of a metal gate MOSFET. In order to provide optimal performance for a metal gate MOSFET, however, a combination of a metal gate material and a gate dielectric is required such that the metal gate material has a work function near a band gap edge of an underlying semiconductor material and the gate dielectric does not suffer from increase in EOT during processing sequences.